plumbing question

I have a problem... hoping one of you do-it-yourselfers out there can help me out. I need to replace the knob to my shower faucet as the handle has broken. I've tried looking for replacement faucet handles at the Home Depot, but the female end of the knob does not fit the male end that sticks out of my shower wall:http://www.geocities.com/dcden87_2/knob.JPG

As you can see, the cross section of the male end is square.

Among the knobs I've seen available, the cross section of the female end is a near-circle, i.e. it has two flattened sides so that it can grip onto the appropriate male end and turn the faucet. I guess you can describe the cross section as being in the shape of a barrel.

Does anyone know if they sell adapters for this sort of thing? Or do I just have the wrong type of faucet handle? My original faucet set was Sterling, but they have discontinued that line.

If all else fails, you may be able to use one of the "universal" replacement knobs. They are available at most supply houses and have a collar and set screw that will fit most stems. That valve of yours is unfamiliar to me and I can't quite make out the diameter of the shaft.